Ferritic-martensitic steel subjected to equal channel angular extrusion

Modified 9Cr-1Mo ferritic-martensitic steel (T91) has been extensively investigated as a structural material for GenIV nuclear reactors and Accelerator Driven Transmutation systems. One attractive characteristic of this steel in these applications is its superior radiation damage tolerance in comparison to typical austenitic stainless steels such as 316L. In some GenIV applications, it also has a significantly higher corrosion resistance. Further improvement of both is necessary if GenIV designs are to become commercially viable. Other work has shown an improvement in radiation damage tolerance via cold rolling or sputtering nanoscale multilayered films. Additionally, corrosion resistance can be improved by homogenizing the microstructure. Further, these changes can improve the strength of the material. However, there has been no fabrication of bulk ultra fine grain ferritic-martensitic steel candidates that might offer these avenues of improvement. This work demonstrates the refinement and homogenization of T91 by Equal Channel Angular Extrusion (ECAE) and heat treatment. Processing temperature and strain level were varied to produce multiple levels of refinement. Materials were characterized by microhardness, tensile testing, x-ray diffraction and transmission electron microscopy. An ultra-fine, highly misoriented and homogeneous microstructure was achieved in the material. Refinement was demonstrated both in ferritic and ferritic-martensitic compositions of the steel. Microhardness increased by as much as 70% and ultimate tensile strength by 80%. More significantly, tensile strength was improved by 40% without decreasing ductility